A tappet is a valve train component which converts rotating motion into linear motion in activating a valve. It is most commonly found in internal combustion engines, which converts the rotating motion of the camshaft into linear motion of intake and exhaust valves, either directly or indirectly.

A poppet valve is a valve typically used to control the timing and quantity of petrol (gas) or vapour flow into or out of an engine, but with many other applications.